MarkWest Energy Partners LP, Denver, (NYSE: MWE) and Houston-based private-equity fund NGP Midstream & Resources LP have amended their joint venture formed in February to construct and operate natural-gas midstream facilities to support producers in the Marcellus shale.

“M&R has been an excellent partner in the Marcellus project,” says Frank Semple, MarkWest chairman, president and chief executive. “Range Resources and other producers continue to experience significant success in the Marcellus and as a result, their need for additional midstream infrastructure continues to grow. This amendment will provide additional funding to allow the joint venture to keep pace with the dramatic increase in volumes and will allow MarkWest to maintain its required capital flexibility.”

MarkWest contributed approximately $100 million at the closing of the joint venture and NGP M&R will have invested some $200 million by the fourth quarter.

Now with the amended agreement, NGP M&R will invest an additional $150 million, which is expected to be the majority of the capital required in 2010. After NGP has contributed the additional $150 million, MarkWest will contribute the majority of the future required capital until MarkWest has invested 51% of the joint venture’s total capital.

MarkWest and NGP M&R will maintain a 60%-40% respective ownership interest until Jan. 1, 2011, when the ownership percentages will be adjusted to 51%-49%, respectively.

MarkWest has established a leading position in providing midstream services in the Marcellus, including the development of gathering and processing infrastructure for Range Resources in southwestern Pennsylvania. MarkWest and NGP M&R expect the joint venture to be capable of processing up to 90 million cu. ft. per day of gas by the end of 2009 and up to 470 million cu. ft. per day in 2011.